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      <title>4、Java NIO 中的 Selector - 交互式测验</title>
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    </head>
    <body>
      <div class="main-wrapper">
          <div class="quiz-container">
            <div class="quiz-header">
              <h1>4、Java NIO 中的 Selector - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
            </div>
            <div id="quiz-content">
                <div class="quiz-body">
                    <div class="progress-bar">
                        <div id="progress-indicator"></div>
                    </div>
                    <div id="question-container">
                      <p id="question-text"></p>
                      <ul class="options-list" id="options-list"></ul>
                    </div>
                    <div id="result-container"></div>
                    <div id="hint-container" class="hint-container"></div>
                </div>
                <div class="quiz-footer" id="quiz-footer">
                  <button id="prev-btn" class="nav-btn">上一题</button>
                  <button id="hint-btn" class="nav-btn" style="background-color: #ffc107; color: #333; box-shadow: 0 4px 12px rgba(255,193,7,0.3);">提示</button>
                  <button id="next-btn" class="nav-btn">下一题</button>
                </div>
            </div>
          </div>
      </div>

      <script>
        const quizData = [
  {
    "question": "与传统的阻塞I/O模型相比，Java NIO的Selector主要解决了什么核心问题？",
    "options": [
      "通过'一个连接一个线程'的模型简化了网络编程的复杂度。",
      "解决了因大量连接导致线程资源消耗过大和上下文切换开销高的问题。",
      "保证了数据在网络传输过程中的绝对可靠性。",
      "显著提高了单个连接的数据传输速度。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "传统的阻塞I/O为每个连接创建一个线程，当连接数巨大时，会创建大量线程，导致内存消耗和CPU上下文切换成本急剧上升。Selector通过事件驱动模型，允许单个线程管理多个连接（Channel），只有当某个连接有事件就绪时才去处理，从而用极少的线程支持大量并发连接。",
    "source": "文档中「1.2 Selector 解决了什么问题？」部分",
    "hint": "文档中提到传统模型的缺点是'线程资源消耗过大'和'线程上下文切换开销增加'。"
  },
  {
    "question": "Java NIO的Selector是如何实现跨平台的I/O多路复用能力的？",
    "options": [
      "它完全在Java虚拟机内部通过一个高效的事件调度器实现，与操作系统无关。",
      "它依赖于操作系统底层的I/O多路复用机制，例如Linux的epoll或Windows的IOCP。",
      "它为每个连接创建一个轻量级的'虚拟线程'来进行监控。",
      "它通过轮询所有已注册的通道来检查其状态，这是一种纯软件层面的实现。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "Selector的实现并非纯Java代码，而是通过JNI（Java Native Interface）调用了操作系统提供的原生I/O多路复用功能。例如，在Linux上它会使用epoll，在macOS上使用kqueue。这使得Selector能够高效地利用操作系统级别的特性来监控大量文件描述符的状态。",
    "source": "文档中「2.1 Selector 的内部实现」部分",
    "hint": "文档中提到了`EPollSelectorImpl`类，并说明其实现依赖于Linux的`epoll`机制。"
  },
  {
    "question": "在Java NIO编程中，`SelectionKey`扮演了什么关键角色？",
    "options": [
      "它是一个用于存储网络数据的缓冲区对象。",
      "它是一个独立的线程，专门用于处理特定Channel的I/O操作。",
      "它是一个代表Channel向Selector注册的句柄，封装了事件信息和通道本身。",
      "它是Selector的替代品，用于管理单个连接。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "`SelectionKey`是连接Channel和Selector的桥梁。当一个Channel注册到Selector上时，就会创建一个`SelectionKey`对象。这个对象不仅包含了对Channel的引用，还记录了该Channel感兴趣的事件类型（如OP_READ, OP_WRITE）以及当前已经就绪的事件类型。",
    "source": "文档中「2.2 SelectionKey：事件与通道的桥梁」部分",
    "hint": "它的标题是'事件与通道的桥梁'，这揭示了它的核心作用。"
  },
  {
    "question": "在处理`selector.selectedKeys()`返回的集合时，为什么必须在处理完每个`SelectionKey`后调用`iterator.remove()`方法？",
    "options": [
      "为了释放`SelectionKey`对象占用的内存，防止内存泄漏。",
      "因为Selector本身不会从selected-keys集合中移除`SelectionKey`，如果不手动移除，下次调用`select()`后会重复处理同一个已就绪的事件。",
      "为了通知Selector该事件已经处理完毕，可以接收新事件。",
      "为了将`SelectionKey`从'感兴趣集合'中移除，停止监听该事件。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "`selector.selectedKeys()`返回的是已就绪事件的`SelectionKey`集合。Selector将就绪的键放入此集合，但不会在处理后自动移除它们。程序员必须在迭代处理时手动调用`iterator.remove()`，否则在下一次循环中，如果该通道仍然就绪，这个键会依然存在于集合中，导致不必要的重复处理。",
    "source": "文档中「3.1 基本使用步骤」部分的示例代码注释",
    "hint": "思考一下如果不移除，下一次`select()`循环会发生什么。代码示例中有一行关键注释：`it.remove(); // 从集合中移除，防止重复处理`。"
  },
  {
    "question": "根据文档中的性能优化技巧，在多核CPU环境下，提升Selector应用性能的有效策略是什么？",
    "options": [
      "将所有Channel都注册到一个Selector上，并增加其运行线程的优先级。",
      "为所有连接共享一个巨大的`ByteBuffer.allocateDirect()`缓冲区。",
      "创建多个Selector实例，每个实例由一个独立的线程驱动，并将新连接轮流分配给这些Selector。",
      "总是使用`selector.select()`无参方法，避免设置超时时间。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "单个Selector由单个线程驱动，无法充分利用多核CPU。一个常见的优化模式（类似Netty中的多Reactor模型）是创建一个Selector池，池的大小通常与CPU核心数相关。主Selector负责接受新连接，然后将新连接（SocketChannel）轮流注册到工作Selector上，由它们各自的线程负责处理后续的读写事件，从而实现负载均衡。",
    "source": "文档中「3.2 性能优化技巧」部分",
    "hint": "文档中提到 '在多核环境下，可以使用多个 Selector 分担负载'，并给出了相应的代码示例。"
  },
  {
    "question": "文档描述了一种可能导致CPU使用率飙升的特定问题：`selector.select()`方法在没有就绪通道的情况下被频繁唤醒。这个问题通常被称为什么？",
    "options": [
      "线程死锁 (Deadlock)",
      "内存泄漏 (Memory Leak)",
      "网络风暴 (Network Storm)",
      "Selector空轮询Bug (Selector empty poll bug)"
    ],
    "correctAnswerIndex": 3,
    "explanation": "这是JDK在某些版本（尤其是在Linux内核上）存在的一个著名bug。正常情况下，`select()`方法应该阻塞直到有事件发生或超时。但此bug会导致`select()`在没有任何事件时立即返回0，如果循环中没有妥善处理，就会形成一个密集的循环，不断调用`select()`，导致CPU 100%。Netty等框架内部都对此bug进行了修复或规避。",
    "source": "文档中「3.2 性能优化技巧」部分",
    "hint": "文档第四个性能优化技巧的标题直接指出了这个问题的名称。"
  },
  {
    "question": "尽管Selector性能强大，但文档指出了直接使用它存在一些局限性。以下哪一项是其最主要的局限性之一？",
    "options": [
      "编程模型复杂，代码难以编写和维护。",
      "与现代操作系统（如Linux 5.x）不兼容。",
      "无法同时处理TCP和UDP连接。",
      "在高并发场景下性能远不如传统的阻塞I/O。"
    ],
    "correctAnswerIndex": 0,
    "explanation": "相比于简单直观的'一个请求，一个线程，同步处理'的阻塞I/O模型，NIO Selector的事件驱动和异步特性要求开发者手动管理连接状态、缓冲区、事件类型切换等，代码逻辑更加复杂，也更容易出错。这也是为什么Netty这类框架会大受欢迎，因为它极大地封装了这些复杂性。",
    "source": "文档中「5.1 Selector 的局限性」部分",
    "hint": "思考一下为什么开发者更倾向于使用Netty而不是直接使用Selector。文档列出的第一条局限性是什么？"
  }
];
        console.log("Quiz Data:", JSON.parse(JSON.stringify(quizData))); // 添加这行用于调试，以可展开的JSON格式输出
        
        let currentQuestionIndex = 0;
        let score = 0;
        // 存储每个问题的回答状态
        const userAnswers = new Array(quizData.length).fill(null); 

        const questionTextEl = document.getElementById('question-text');
        const optionsListEl = document.getElementById('options-list');
        const resultContainerEl = document.getElementById('result-container');
        const quizContentEl = document.getElementById('quiz-content');
        const progressIndicatorEl = document.getElementById('progress-indicator');
        const prevBtn = document.getElementById('prev-btn');
        const nextBtn = document.getElementById('next-btn');
        const hintBtn = document.getElementById('hint-btn');
        const hintContainerEl = document.getElementById('hint-container');

        function loadQuestion() {
          resultContainerEl.innerHTML = '';
          optionsListEl.innerHTML = '';
          hintContainerEl.style.display = 'none'; // 隐藏提示
          
          if (currentQuestionIndex >= quizData.length) {
            showFinalScore();
            return;
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          const currentQuestion = quizData[currentQuestionIndex];
          questionTextEl.textContent = `(${currentQuestionIndex + 1}/${quizData.length}) ${currentQuestion.question}`;
          
          currentQuestion.options.forEach((optionText, index) => {
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            li.className = 'option-item';
            li.textContent = optionText;
            li.dataset.index = index; // Store index in a data attribute
            // 如果这道题已经回答过，恢复状态
            if (userAnswers[currentQuestionIndex] !== null) {
                if (index === userAnswers[currentQuestionIndex]) {
                    li.classList.add('selected');
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            optionsListEl.appendChild(li);
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          // 如果已经回答过，直接显示结果
          if (userAnswers[currentQuestionIndex] !== null) {
              const isCorrect = userAnswers[currentQuestionIndex] === currentQuestion.correctAnswerIndex;
              showResult(isCorrect, currentQuestion);
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          updateNavigationButtons();
          updateProgress();
        }

        function selectOption(optionIndex) {
            userAnswers[currentQuestionIndex] = optionIndex;
            const currentQuestion = quizData[currentQuestionIndex];
            const isCorrect = optionIndex === currentQuestion.correctAnswerIndex;

            // 重新计算分数
            score = userAnswers.reduce((acc, answer, index) => {
                return answer === quizData[index].correctAnswerIndex ? acc + 1 : acc;
            }, 0);

            showResult(isCorrect, currentQuestion);
            updateNavigationButtons();
        }
        
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            const answeredQuestions = userAnswers.filter(answer => answer !== null).length;
            const progressPercentage = (answeredQuestions / quizData.length) * 100;
            progressIndicatorEl.style.width = `${progressPercentage}%`;
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        function updateNavigationButtons() {
            prevBtn.style.display = 'block';
            nextBtn.style.display = 'block';
            hintBtn.style.display = 'block'; // 显示提示按钮

            prevBtn.disabled = currentQuestionIndex === 0;
            nextBtn.disabled = false; // 最后一题时，下一题按钮不禁用，而是用于显示报告
            nextBtn.textContent = (currentQuestionIndex === quizData.length - 1) ? '查看报告' : '下一题';
            
            // 如果当前问题没有提示，则禁用提示按钮并隐藏提示内容
            if (!quizData[currentQuestionIndex].hint) {
                hintBtn.disabled = true;
                hintContainerEl.style.display = 'none';
                hintContainerEl.textContent = ''; // 清除旧的提示内容
            } else {
                hintBtn.disabled = false;
            }
        }

        function showResult(isCorrect, questionData) {
            resultContainerEl.innerHTML = ''; // 清空之前的结果
            const resultBox = document.createElement('div');
            resultBox.className = `result-box ${isCorrect ? 'correct' : 'incorrect'}`;
            
            let html = `<h3>${isCorrect ? '回答正确！' : '回答错误'}</h3>`;
            if (!isCorrect) {
                html += `<p><strong>正确答案是：</strong> ${questionData.options[questionData.correctAnswerIndex]}</p>`;
            }
            html += `<div class="explanation-text">${questionData.explanation}</div>`;
            html += `<div class="source-text">来源: ${questionData.source}</div>`;
            
            resultBox.innerHTML = html;
            resultContainerEl.appendChild(resultBox);

            // 禁用所有选项的点击事件，并高亮正确/错误答案
            Array.from(optionsListEl.children).forEach((child, index) => {
                child.style.pointerEvents = 'none';
                // 高亮正确答案
                if (index === questionData.correctAnswerIndex) {
                    child.classList.add('correct'); // 需要添加CSS样式
                    child.style.backgroundColor = '#d4edda';
                    child.style.borderColor = '#c3e6cb';
                }
                // 如果选错了，高亮错误选项
                if (!isCorrect && index === userAnswers[currentQuestionIndex]) {
                    child.classList.add('incorrect'); // 需要添加CSS样式
                    child.style.backgroundColor = '#f8d7da';
                    child.style.borderColor = '#f5c6cb';
                }
            });
        }

        function goToNextQuestion() {
            if (currentQuestionIndex < quizData.length - 1) {
                currentQuestionIndex++;
                loadQuestion();
            } else {
                showFinalScore();
            }
        }

        function goToPreviousQuestion() {
            if (currentQuestionIndex > 0) {
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                loadQuestion();
            }
        }
        
        function showHint() {
            const currentQuestion = quizData[currentQuestionIndex];
            if (currentQuestion.hint) {
                hintContainerEl.textContent = currentQuestion.hint;
                hintContainerEl.style.display = 'block';
            } else {
                hintContainerEl.style.display = 'none';
            }
        }

        function showFinalScore() {
            document.querySelector('.quiz-body').style.display = 'none';
            document.getElementById('quiz-footer').style.display = 'none';

            const correctCount = score;
            const totalQuestions = quizData.length;
            const answeredCount = userAnswers.filter(a => a !== null).length;
            const wrongCount = answeredCount - correctCount;
            const skippedCount = totalQuestions - answeredCount;
            const accuracy = totalQuestions > 0 ? Math.round((correctCount / totalQuestions) * 100) : 0;
            const errorRate = totalQuestions > 0 ? Math.round((wrongCount / totalQuestions) * 100) : 0;

            const finalScoreHtml =
                '<div class="final-score-container">' +
                '<h2>答题报告</h2>' +
                '<div class="score-summary">' +
                '<div class="score-card">' +
                '<h3>得分</h3>' +
                '<p>' + correctCount + ' / ' + totalQuestions + '</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>正确率</h3>' +
                '<p>' + accuracy + '%</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>错误率</h3>' +
                '<p>' + errorRate + '%</p>' +
                '</div>' +
                '<div class="score-card score-details">' +
                '<table>' +
                '<tr><td>正确</td><td>' + correctCount + '</td></tr>' +
                '<tr><td>错误</td><td>' + wrongCount + '</td></tr>' +
                '<tr><td>未答</td><td>' + skippedCount + '</td></tr>' +
                '</table>' +
                '</div>' +
                '</div>' +
                '<div class="report-actions">' +
                '<button id="review-btn">回顾测验</button>' +
                '<button class="primary" onclick="location.reload()">再试一次</button>' +
                '</div>' +
                '</div>';

            let reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }
            quizContentEl.insertAdjacentHTML('beforeend', finalScoreHtml);
            document.getElementById('review-btn').addEventListener('click', reviewQuiz);
        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  